Not coming all the way down to set idle

I assume you have the port at the bottom front (that shows in your pix) capped off? As well as other unused base ports?

If the prior carb idled down OK, then I'd suspect that the secondary stop screw is open more than it should be. Or there are holes drilled in the primary plates.

Otherwise, I would also be looking at the PCV flow at idle vacuum. Pull it with the hose attached and put your finger over the opening on the bottom of the PCV and see if the engine idle drops. What vacuum level and waht PCV are you using?